The present invention relates to a vacuum envelope for a display device such as a cathode ray tube used mainly for television broadcast signal reception or an industrial equipment, or a field emission display unit (herein-below, referred to as FED).
In a conventional display device such as a cathode ray tube, FED or the like wherein phosphors are excited to emit light by utilizing a kinetic energy of electrons moving at a high speed under high vacuum condition, it was difficult to form a portion which directly contacts the inside of the vacuum envelope, with a resinous material because the resinous material is poor in hermetic properties even though it has an advantage of having a low density. It has been considered to be indispensable for a material used for the inside of the vacuum envelope to use glass from the viewpoints of a mechanical strength capable of withstanding an atmospheric pressure, X-ray absorbing properties, electrical resistance properties, heat resistance properties in manufacturing process, a risk of causing a damage by electron beams, and so on, in addition to the necessity of maintaining high vacuum condition.
In a typical cathode ray tube as shown in FIG. 3, a vacuum envelope or a glass bulb 2 is composed of a panel portion 3 for displaying an image and a funnel portion 4 including a neck portion 5 in which an electron gun 17 is housed.
In FIG. 3, reference numeral 6 designates a panel skirt portion, numeral 7 a panel face for displaying an image, numeral 8 an implosion protection reinforcing band for providing a sufficient strength, numeral 10 a sealing portion for sealing the panel portion 3 to the funnel portion 4 with a solder glass or the like, numeral 12 phosphors for emitting fluorescence by the irradiation of electron beams, numeral 13 an aluminum film which reflects the fluorescence forwardly, numeral 14 a shadow mask for landing electron beams to predetermined positions on the phosphors, and numeral 15 a stud pin for fixing the shadow mask 14 to an inner wall of the panel skirt portion 6. A character A indicates a tube axis which extends through a center line of the neck portion 5 and a center line of the panel portion 3.
The cathode ray tube is so adapted as to display an image by impinging electron beams to the phosphors at a high speed in an inner space of the vacuum envelope to excite the phosphors whereby light is emitted. Accordingly, the inside of the vacuum envelope is maintained to be high vacuum condition of about 10xe2x88x928 Torr. Since the cathode ray tube has an asymmetric structure different from a spherical shape, one atmospheric pressure as a pressure difference between the inside and the outside of the vacuum envelope is applied thereto. Therefore, there is always a high deformation energy in the vacuum envelope and it is in an unstable state with respect to deformation.
If a crack is produced in the glass bulb of the A cathode ray tube in such an unstable state, a force to release an existing high deformation energy is resulted whereby the crack will develop to invite fracture. Further, in the state that a high tensile stress is loaded in an outer surface of the cathode ray tube, a delayed fracture is resulted from stress corrosion due to moisture in air, which also loses the reliability. From the above-mentioned reasons, there is a requirement to increase the thickness of a glass bulb so that a sufficient mechanical strength can be provided. As a result, for example, the weight of a glass bulb with about 29-inch screen diameter at a diagonal axis becomes to about 25 kg.
On the other hand, several kinds of image displaying devices other than cathode ray tubes have recently been proposed. It is well-known that disadvantages of the cathode ray tubes in comparison with these image-displaying devices reside mainly in that the depth and the weight of such displaying devices are large. Accordingly, attempts to shorten the depth or reduce the weight have been made.
In a conventional cathode ray tube, when the depth is shortened, a degree of asymmetry of structure of the cathode ray tube is increased, and there creates a problem that a further amount of deformation energy is accumulated in the vacuum envelope. Further, in an attempt to reduce the weight, a deformation energy is generally increased owing to reduction in the rigidity of glass. The increase of the deformation energy will increase stresses. Accordingly, reduction in safety due to fracture and reduction in reliability due to a delayed fracture are accelerated. When the wall thickness of the glass is increased in order to prevent the increase of stresses, the weight is inevitably increased.
In a typical FED as shown in FIG. 4, the vacuum envelope is basically composed of a front panel 23 made of glass for displaying an image, a rear panel 24, as a substrate for an electron emitting source, which emits electrons in an field emission mode, and an outer frame 25. Reference numeral 26 designates a cathode on which an electron emitter 27 is formed. A gate electrode 28 is formed on the rear panel 24 by interposing an insulation layer 29 so that the gate electrode controls an electron current. An anode 30 is formed on the front panel 23 and pixels 31 are formed on the anode 30 so that each pixel corresponds to each electron emitter 27. The front panel 23 and the rear panel 24 are connected with the outer frame 25 around of which is hermetically sealed with a solder glass or the like. An inner space surrounded by these members is maintained to be high vacuum condition exceeding 10xe2x88x928 Torr.
Accordingly, FED should have a structure durable to an atmospheric pressure in the same manner as the cathode ray tube. Each of the wall thickness of the front panel 23 and the rear panel 24 both made of glass have to be increased in order to assure a predetermined strength. Accordingly, the weight of the vacuum envelope is fairly increased.
Heretofore, there was proposed in a publication (JP-A-8-007793) to provide a reinforcing member made of resin on an outer surface of the glass bulb in order to reduce the weight of the vacuum envelope used for a cathode ray tube, wherein the reinforcing member made of resin has a smaller density than glass. In general, the wall thickness of the panel face center of a 29-inch model glass bulb is about 14-15 mm. In the publication, however, there is description that the wall thickness of the glass panel is, for example, 7-8 mm and the thickness of polycarbonate as a reinforcing member of plastics is the same, i.e., 7-8 mm in an example. Generally, the density of the glass panel for a color cathode ray tube is about 2.8 g/cm3 and the density of the polycarbonate is about 1.1 g/cm3. Accordingly, a reduction of weight of about 30% can be achieved.
However, the Young""s modulus of the glass panel is 7000-8000 kgf/mm2 whereas the Young""s modulus of polycarbonate is about 240 kgf/mm2, which is about {fraction (1/30)} of the Young""s modulus of the glass panel. Accordingly, when a load of atmospheric pressure is applied to a vacuum envelope having the above-mentioned structure, the maximum tensile stress produced in an outer surface at an edge portion of a screen area of the glass panel of the vacuum envelope is about twice as much as the maximum tensile stress produced in a vacuum envelope of single-layered structure. Namely, when such a complex-layered structure is used, a tensile stress beyond the strength of glass in practical use is resulted from a difference in inner and outer pressures in an outer surface of the glass bulb whereby there is a possibility of causing breakage.
Further, in the case of an atmospheric pressure being applied, an amount of deflection produced in the vacuum envelope having a complex-layered structure becomes about three times as much as that produced in a vacuum envelope having a single-layered structure. As a result, correct relative positions between the positions of the phosphors and landing positions of electron beams may not be assured, whereby desirable operations for a display may not be expected. Thus, the complex-layered structure according to the conventional technique could not achieve a large reduction of the weight of the vacuum envelope while the mechanical strength of the vacuum envelope and a necessary precision for the operations of a display can be maintained in a practically usable range.
Even in FED adapted to emit electrons under high vacuum condition to excite phosphors so that light is emitted, a vacuum envelope made of glass is used in the same manner as the cathode ray tube. A publication (JPA-10-188857) proposes a technique to reduce the weight of the vacuum envelope for FED. Namely, the vacuum envelope has a structure formed by opposing two thin panels of glass with a predetermined distance; the peripheral portions of the thin panels of glass are sealed, and a sealed inner space is vacuumed, wherein a reinforcing sheet is formed integrally with a rear face of at least one of the thin panels so as to be durable to an atmospheric pressure, the reinforcing sheet being of a material having a larger Young""s modulus than that of a material for the thin panels.
However, the publication describes that materials having a larger Young""s modulus and a smaller density than those of glass, by which the reduction of the weight can be achieved, are ceramics such as silicon nitride, zirconia, alumina, or the like. These materials are opaque in a wave range of visible light. This means that materials having a larger Young""s modulus than glass are unsuitable in terms of optics as the reinforcing member used for the screen area. On the other hand, a transparent methacrylate resin has a lower density such as about 1.2 g/cm3. However, it does not provide a sufficient rigidity because the Young""s modulus is small as about 260 kgf/mm2. Accordingly, it is unsuitable for the reinforcing member.
It is an object of the present invention to provide a vacuum envelope for a cathode ray tube or FED, which can reduce the weight without causing a substantial increase of stress and deflection and which is safe and reliable.
In accordance with the present invention, there is provided a vacuum envelope for display which has a substantially rectangular screen area, wherein the screen area is essentially composed of a complex layer member comprising at least glass as an inner layer which contacts the exhausted inner portion and a transparent resin as an outer layer, and the proportion of the Young""s modulus EP of the transparent resin to the Young""s modulus EG of the glass is {fraction (1/10)}-⅕.
Further, according to the present invention, there is provided the vacuum envelope as described above, wherein the proportion of EP to EG is {fraction (1/10)}-{fraction (1/7)} and xcfx81P3/EP is smaller than xcfx81G3/EG where xcfx81P represents the density of the transparent resin and xcfx81G represents the density of the glass.